This invention relates to photoflash lamps and, more particularly, to an improved protective coating for flashlamps.
A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the continuing effort to improve light output, higher performance flashlamps have been developed which contain higher combustible fill weights per unit of internal envelope volume, along with higher fill gas pressure. In addition, the combustible material may be one of the hotter burning types, such as hafnium. Such lamps, upon flashing, appear to subject the glass envelopes to more intense thermal shock effects, and thus require stronger containment vessels. One approach to this problem has been to employ a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385, along with a protective dip coating of cellulose acetate. Although providing some degree of improvement in the containment capability of lamp envelopes, the use of cellulose acetate dip coatings and hard glass present significant disadvantages in the areas of manufacturing cost and safety. More specifically, the hard glass incurs considerably added expense over the more commonly used soft glass due to both increased material cost and the need for special lead-in wires to provide sealing compatibility with the hard glass envelope. In addition, even though more resistant to thermal shock, hard glass envelopes can also exhibit cracks and crazes upon lamp flashing, and, thus, do not obviate the need for a protective coating.
Another approach toward providing an improved containment vessel for photoflash lamps has been to employ a stronger, more temperature resistant coating material on the exterior of the glass envelope. For example, U.S. Pat. No. 3,156,107 describes a flashlamp having an exterior coating of polycarbonate resin, a material which exhibits relatively high impact and tensile strengths and a high softening temperature.
Yet a further approach to providing a more economical and improved containment vessel is described in a copending application Ser. No. 268,576, filed July 3, 1973 now Pat. No. 3,893,797 and assigned to the assignee of the present application. According to this previously filed application, a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope. The method of applying the coating comprises: placing the glass envelope within a preformed sleeve of the thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the glass envelope; and, simultaneosly heating the assembly incrementally along its length, whereby the temperature and vacuum cause the thermoplastic to be incrementally formed onto the glass envelope with the interface substantially free of voids, inclusions and the like. This method provides an optically clear protective coating by means of a significantly faster, safer and more economical manufacturing process, which may be easily integrated on automated production machinery.
Heat is employed in applying the polycarbonate resin coatings on the lamp envelopes. Subsequent cooling of the glass envelope and polycarbonate coating causes the buildup of high tensile forces in the coating because it tends to contract more than the glass. These forces can be reduced somewhat by heating a narrow band of the coating as described in U.S. Pat. No. 3,832,257. It has been found, however, that even such stress relieved coatings can crack and fail in a relatively short time under conditions of high humidity, even when the remaining stresses are within the accepted design limits for the polycarbonate resin used. It should be noted here that unstressed polycarbonate has good resistance toward humidity or even water immersion. In searching for a solution to this aging, or shelf-life, problem under humid conditions, an extensive literature survey failed to shed light on the cause of this unexpected cracking under stress levels allowed by good design practices.
Consideration was then given to the incorporation of a compatible plasticizer into the resin with the anticipation that it might promote relaxation and stretching and thereby relieve the stresses caused by differential contraction between the coating and glass. Evaluation of coatings containing, for example, 20 or 30 parts of a plasticizer to 100 parts of resin did in fact show significantly improved life under humid conditions. The plasticized polycarbonate was quite rigid rather than extensive as had been expected, and therefore, did not function in the manner anticipated. That is, the reduced coating stresses obtained with the plasticized resin where the result of a considerable lowering of the softening temperature needed for thermoforming. Cooling of the coated lamp over a lesser temperature gradient resulted in less stress build up. The shortcoming of the approach, however, was that the introduction of the required amounts of plasticizer resulted in substantial weakening of the coating, when compared to unplasticized polycarbonate. The resulting plasticized polycarbonate did not provide the desired stronger protective coating; more specifically, the plasticized polycarbonate coatings were not consistently better than cellulose acetate lacquer in containment tests with overcharged lamps. In addition, with respect to the preformed polycarbonate sleeves which are vacuum-formed onto the lamp, the low set point and poor strength at elevated temperatures of the plasticized polycarbonate made extraction of the injection molded sleeves from the mold a difficult, slow and uneconomical process.